CpSc 875
Project 1
Embedded Control Systems
Anti-Lock Braking System
Architecture
Documentation-Volume 1
Team
Sai Madhavapeddi
Preetham K Yellambalase
February 2004
1
2
Table of Contents
1
ABS Architecture Documentation Roadmap
5
2
ABS System Overview
6
3
ABS Architecture View Template
8
4
Mapping Between Views
10
5
Directory
11
6
Architecture Glossary and Acronym List
12
7
Rationale, Background, and Design Constraints
14
8
Process for modifying the Architecture
15
9
References and Further Reading
16
3
List of Figures
Figure 1- Attached Process for Change of Requirement
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1 ABS Architecture Documentation Roadmap
1.1 Introduction
This document describes our architecture for the Anti-lock braking system in an
automobile. The document follows the format described in [clements 03]
The Anti-lock braking system is a common safety feature implementation in today’s
automobile. The ABS system is designed to prevent loss of steering control and skidding
associated with hard braking situations. The wheel lockup is the chief cause for this kind
of skidding. The principle behind the working of the ABS system is the transfer of brake
fluid pressure into a secondary reservoir when the skidding is imminent and restoring it
back when the pressure decreases. It is adopted when the deceleration rate for a given
wheel breaches the threshold rate. ABS system helps in achieving maximum braking
force and complete steering control.
1.2 Contents
This volume, volume 1, provides information that supports the actual architecture model,
which is shown in volume 2.
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2 ABS System Overview
Safety is of utmost importance in all modern vehicle designs. This has caused for
innovations such as adaptive cruise control, night vision technology, air bag technology,
and anti-lock braking systems. All of these systems exploit embedded software to reduce
cost, and allow complex control with respect to a given system.
2.1 History
ABS was first invented and applied in the aircraft industry and then was introduced to
automobile industry in the early 1970's. However, it had not been used popularly until the
middle of the 1980's due to technical difficulties and high cost.
Ford gets the prize for being the first company to embrace RABS (Rear Anti-lock
Braking System -- at less than $100 per vehicle for about 80% of the benefit of a fourwheel system, it was a safety bargain). In '87, it appeared on F-series pickups, Broncos,
and Bronco II's as standard equipment. Chevy followed with a similar system on its
redesigned '88 C-series, and calls it RWAL (Rear Wheel Anti-Lock).
This architecture for the ABS system was selected among the various other candidate
architectures by the architecture team consisting of Sai and Preetham.
2.2 Description
ABS functions in place of the traditional brake system at times of wheel lock-up. A quick
test sequence checks all the components of the system. If ever the test sequence fails, the
normal brake system is in control. Although the normal brake system can give instant and
efficient braking, it can cause the wheels to be lock up, therefore, the driver can not steer
and would lose control of the car. If any of the wheels happen to be skidding, the driver
must recognize wheel-skid and manually 'pump the brakes' to avoid a skid. The
advantage of ABS lies in its ability to allow the driver retain steering control in order to
keep the car moving in the direction that the wheels are turned towards, rather than
skidding
in
the
direction
of
the
car's
forward
momentum.
ABS has the classic design of an embedded system.



controller.
sensor, wheel speed.
actuators (valve and ABS reservoir) at each wheel.
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ABS functions independently at each wheel. The control unit gets the speed via
wheelspeed sensors at each wheel in order to calculate the deceleration. If there exists a
case of rapid deceleration, ABS will be activated prior to wheel lock-up. Pressure on
caliper is released by opening a valve, which interfaces with the wheel cylinder, letting
oil flow into a small reservoir.
Anti-lock brakes function well on wet paved surfaces and icy or packed snow-covered
roads. Stopping times will be longer on gravel or fresh snow, although operators will not
experience the dangerous lockup of wheels and loss of steering control usually associated
with conventional braking.
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3 ABS Architecture View Template
We adopt the View Template presented in [Clements 03]. In this section we briefly
describe each section in the template. The template is then used in Volume 2 to organize
each View Packet.
3.1 View Packet Template Description
3.1.1 Primary presentation
This provides the basic model for this part of the architecture.
3.1.2 Element Catalog
Each element included in a view is described in the element catalog. Each entry in the
catalog follows this outline:
3.1.2.1 Properties of the elements
An element may have specific properties that affect the system’s ability to reach desired
levels in its quality attributes. For example, an element may have a large negative impact
on the performance of the overall system.
3.1.2.2 Relations and their properties
Elements will have relationships with other elements. One element may aggregate
another element or it may be a specialization of another element.
3.1.2.3 Element Interfaces
The element’s interface describes its publicly available services. This information is
provided using method signatures.
3.1.2.4 Element Behavior
The element’s interface description does not fully describe its behavior. That behavior is
described using UML dynamic diagrams.
3.1.3 Context diagram
This presentation places the elements contained in the view packet in the larger context of
the overall architecture.
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3.1.4 Variability guide
In this section we describe the variations that are possible within this view.
3.1.5 Architecture background
3.1.5.1 Rationale
Provides the decisions made that resulted in the current shape of the architecture.
3.1.5.2 Analysis results
This section provides the data that backs up the decisions.
3.1.5.3 Assumptions
Any basic assumptions that underlie the decisions
3.1.6 Other information
Whatever else we want to say but didn’t think it belonged in a specific section.
3.1.7 Related view packets
View packets surrounding this one at the same level of detail,
view packets that include this view at a higher level, or
view packets that describe additional elements within this view packet.
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4 Mapping Between Views
4.1 General mapping
In general, we are using UML to document the architecture. In UML there are static
diagrams that describe definitional units, such as classes, and dynamic diagrams that
describe operational units such as objects. Any operational unit that is used must
correspond to a definitional unit. Therefore, there is a general mapping from dynamic
diagrams to static diagrams.
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5 Directory
All of the information about the ABS system is on the web:
http://www.geocities.com/preethamyk/Group1ATAMFinalReport.doc
http://www.geocities.com/preethamyk/architecturedetail.doc
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6 Architecture Glossary and Acronym List
Data Input Interface
 Receives the state of the brakes from the Brake module. If the brakes are engaged, it
receives a brake-on signal and if the brakes are not engaged, it receives a brake-off signal.
 Receives the rotational speed data from the four wheel speed sensors module.
 Receives signals from the ignition module. When the vehicle’s engine is turned on, it gets
a power-on signal and when it is switched off, it receives a power-off signal.
 Receives the test results of the initial test and the test conducted when the brakes are
applied, from the feedback module.
 Receives the current velocity of the vehicle from the wheel speed module.
This module sends all these signals that it receives to the main program module where all
the processing takes place.
Brake Module
This module is responsible for providing the signal to the data input module about the state of the
brakes. It sends a brake-on signal if the brakes are engaged and sends a brake-off signal if the
brakes are no longer engaged.
Wheel Speed Sensor Module
It sends the current velocity of the vehicle to the data input module. Magnetic sensors are
attached to each wheel. This way, the data is collected from all the 4 wheels and sent to the data
input module.
Ignition Module
When the driver turns the engine on, this module sends a power-on signal to the data input
module and when the engine is turned off, it sends a power-off signal. Also, when the technician
repairs the malfunctioning component, he resets the error lamp bringing back the system.
Tester Module
When the driver turns on the engine, this module conducts an initial test to ensure that all
components are working and sends the test results to the data input module. Similarly, when the
brakes are applied, it conducts a diagnostic test to check for the proper functioning of all the
components of the ABS system, and sends the test results to the data input module.
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ABS Controller Module
This module is like a controller where most of the calculations and decision making takes place.
It receives the various inputs from the data input module. The test results obtained from the full
system test is analyzed. If the test result is a fail, then an error signal is sent to the data output
module. The results from the diagnostic test conducted when the brakes are applied, is also
analyzed. If the test result is a fail, an error signal is sent to the data output module. If the test
result is a pass, the velocity of the vehicle is checked to see if it is below the threshold value of
15 mph. If it is below 15 mph, then ABS is not initiated and only conventional braking applies.
A signal informing about this is sent to the data output module. If the velocity is equal to or
greater than the threshold, the rotational speed data from the 4 wheel speed sensors is used to
calculate the deceleration. If the deceleration threshold of 0.98*9.8 m/s^2 is breached, the wheel
locking will take place and skidding becomes imminent. In this situation, this module sends a
signal to the data output module. Also, when the wheel locking no longer applies, another signal
is sent to the data output module informing the same.
Data Output Module




Sends the Full system test fail signal to the Error Lamp Module
Sends Basic system fail signal to the Error Lamp Module
Sends the error log information to the main computer
Sends pump brake signal to the Fluid Reservoir module
The data output module analyzes the system test fail signals and sends an appropriate signal
depending on the type of error to the error lamp module. This module also analyzes the signal
regarding the pressure of the brake fluid and sends an increase-pressure signal or a decreasepressure signal to the Fluid Reservoir module.
Fluid Reservoir Module
This module receives an increase-pressure signal or a decrease pressure signal from the data
output module. Every wheel has two valves, a pressure increase valve and a pressure decrease
valve. The braking pressure is controlled by activating one of the two. One valve increases the
pressure, up to the maximum, which is the pressure the driver is currently exerting. The other
valve bleeds the fluid off into a secondary reservoir during skid situations to decrease the
pressure. The pressure increase valve is completely opened when conventional braking is in use.
Error Lamp Module
This module receives the error signals from the data output module, which are the formatted
results of the initial test and the test conducted when the brakes are applied. These results are
displayed on the main computer to enable the technician to understand the cause of the error so
that he can take the required components to the service station and get them repaired. Also, an
error lamp is turned on the driver’s console and the ABS system execution is terminated.
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7 Rationale, Background, and Design
Constraints
The ABS system is
1. An event driven system that responds to signals sent by individual modules that
constitute it.
2. A hard-real time system, as lives are at stake if failure occurs.
3. Modular, as each part of the system is responsible for a certain critical feature.
Each module sends out signals or events to other Modules that are connected to it.
4. Modularity also allows the integration of Commercial Off The Shelf (COTS)
components as each module can be realized by a commercially available software
unit.
The major business qualities driving the development of the Anti-Lock Braking System
are as follows:
1. The system must be able to accept commercially available software components.
These components can range from a complete subsystem to a single unit of
implementation in the system. This would indirectly benefit the time to market for
the system.
2. The system should be able to handle upgrades in hardware such as a change in
controller, and also in software. This would enhance the systems modifiability.
3. The system should be built in such a way that it could be deployed on vehicles
from a broad range of makes and models. Although this would increase the time
to develop the system, it would enhance the market area for the product.
4. The system must be able to interact with a wide range of other systems that
constitute an automobile.
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8 Process for Modifying the Architecture
There are several reasons why the architecture will change. In this section we address the
case where a change in requirements occurs.
New requirement
Conduct Change
Impact Analysis
Evolution plan
Select and apply
tactics
Modified Architecture
Conduct
ATAM
Validated Architecture
Figure 1 - Attached Process for Change of Requirement
When a requirement is added or an existing requirement is modified, the impact of that
change to the requirements model is determined. The architecture is reviewed to
determine whether the current architecture can satisfy the new requirement. If not, then
existing elements are examined to determine whether they can satisfy the behaviors. If
not, then element(s) are designed with responsibility for the new behavior. Any new
element(s) are placed in the context of the other elements.
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9 References and Further Reading
The references over the web are as follows
[1] http://www.cs.clemson.edu/~johnmc/productLines/example/adVol1.pdf
[2] http://www.cs.clemson.edu/~johnmc/courses/cpsc875/projects/ABSRequirements.html
[3] http://www.howstuffworks.com/anti-lock-brake.htm
[4] http://www.intel.com/design/auto/mcs96/applnots/27259501.pdf
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